1. Field of the Invention
This invention relates generally to Stirling cycle machines and more particularly to groups of beta free piston Stirling cycle engines and beta free piston Stirling cycle coolers that are balanced to prevent or minimize vibration.
2. Description of the Related Art
Stirling machines have been known for nearly two centuries but in recent decades have been the subject of considerable development because they offer important advantages. Modern versions have been used as engines and heat pumps for many years in a variety of applications. In a Stirling machine of the type used in the invention, a working gas is confined in a working space comprised of an expansion space and a compression space. The working gas is alternately expanded and compressed in order to either do work or to pump heat. Each Stirling machine has a pair of pistons, one referred to as a displacer and the other referred to as a power piston and often just as a piston. The reciprocating displacer cyclically shuttles a working gas between the compression space and the expansion space which are connected in fluid communication through a heat accepter, a regenerator and a heat rejecter. The shuttling cyclically changes the relative proportion of working gas in each space. Gas that is in the expansion space, and/or gas that is flowing into the expansion space through a heat exchanger (the accepter) between the regenerator and the expansion space, accepts heat from surrounding surfaces. Gas that is in the compression space, and/or gas that is flowing into the compression space through a heat exchanger (the rejecter) between the regenerator and the compression space, rejects heat to surrounding surfaces. The gas pressure is essentially the same in both spaces at any instant of time because the spaces are interconnected through a path having a relatively low flow resistance. However, the pressure of the working gas in the work space as a whole varies cyclically and periodically. When most of the working gas is in the compression space, heat is rejected from the gas. When most of the working gas is in the expansion space, the gas accepts heat. This is true whether the machine is working as a heat pump or as an engine. The only requirement to differentiate between work produced or heat pumped, is the temperature at which the expansion process is carried out. If this expansion process temperature is higher than the temperature of the compression space, then the machine is inclined to produce work so it can function as an engine and if this expansion process temperature is lower than the compression space temperature, then the machine will pump heat from a cold source to a warm heat sink.
Stirling machines can therefore be designed to use the above principles to provide either: (1) an engine having a piston and displacer driven by applying an external source of heat energy to the expansion space and transferring heat away from the compression space and therefore capable of being a prime mover for a mechanical load, or (2) a heat pump having the power piston (and sometimes the displacer) cyclically driven by a prime mover for pumping heat from the expansion space to the compression space and therefore capable of pumping heat energy from cooler mass to a warmer mass. The heat pump mode permits Stirling machines to be used for cooling an object in thermal connection to its expansion space, including to cryogenic temperatures, or heating an object, such as a home heating heat exchanger, in thermal connection to its compression space. Therefore, the term Stirling “machine” is used to generically include both Stirling engines and Stirling heat pumps.
Until about 1965, Stirling machines were constructed as kinematically driven machines meaning that the piston and displacer are connected to each other by a mechanical linkage, typically connecting rods and crankshafts. The free piston Stirling machine was then invented by William Beale. In the free piston Stirling machine, the pistons are not connected to a mechanical drive linkage. A free-piston Stirling machine is a thermo-mechanical oscillator and one of its pistons, the displacer, is driven by the working gas pressure variations and differences in spaces or chambers in the machine. The other piston, the power piston, is either driven by a reciprocating prime mover when the Stirling machine is operated in its heat pumping mode or drives a reciprocating mechanical load when the Stirling machine is operated as an engine. Free piston Stirling machines offer numerous advantages including the ability to control their frequency, phase and amplitude, the ability to be hermetically sealed from their surroundings and their lack of a requirement for a mechanical fluid seal between moving parts to prevent the mixing of the working gas and lubricating oil.
Because free-piston Stirling machines can be constructed and operated as an engine, such engines have been linked as a prime mover to a variety of mechanical loads. These loads include linear electric alternators, compressors and fluid pumps and even Stirling heat pumps. Similarly, because free-piston Stirling machines can be operated in a heat pump mode, they have been driven as a load by a variety of prime movers, including linear motors.
Consequently, a Stirling machine, like a linear motor or alternator, are energy transducers that can each be operated in either of two modes. A Stirling machine can be driven mechanically in reciprocation by a prime mover to pump heat from a lower temperature mass to a higher temperature mass. A Stirling machine can be driven by the energy of the temperature difference between two masses and provide an output of mechanical reciprocation. Similarly, a linear motor or alternator structure can be mechanically driven in reciprocation by a prime mover to generate electrical power output or a linear motor/alternator be driven by a source of alternating electrical power to operate as a motor providing a mechanical reciprocating output. Therefore, a Stirling machine operating as an engine can be used to drive a linear alternator and a linear motor can be used to drive a Stirling machine operating in a heat pumping mode. In both of these cases, the power piston of the Stirling machine is ordinarily directly connected to the reciprocating member of the linear motor or alternator so that they reciprocate as a unit.
Stirling machine have been developed in a variety of configurations. A common form of the modern Stirling engine is the alpha configuration, also referred to as the Rinia, Siemens or double acting arrangements. A second Stirling configuration is the beta Stirling configuration characterized by a displacer and piston in the same cylinder. The third is the gamma Stirling configuration characterized by locating the displacer and piston in different cylinders. The present invention deals with beta configuration, free-piston Stirling machines.
Beta FPS machines have reciprocating masses which are principally the power piston, the displacer and structures attached to each of them that reciprocate with each of them. Consequently, there are two reciprocating composite masses in a beta FPS machine that reciprocate along the axis out of phase with each other. The masses are the composite mass of the piston together with structures that are fixed to the piston and therefore reciprocate with the piston and the composite mass of the displacer together with structures that are fixed to the displacer and therefore reciprocate with the displacer. The oscillating acceleration and deceleration of the composite masses of each machine create an axial force (F=ma) alternating between opposite axial directions. These axially alternating forces cause axially oscillating vibration. Because the two composite masses reciprocate along the same axis, they create a resultant axial force alternating between opposite axial directions. Because a resultant axial force is created, for purposes of the explanation of the present invention and discussion of the invention, a FPS machine can be thought of as simply a machine having a single resultant mass reciprocating inside it and along a longitudinal axis. For that reason, FPS machines can be and are symbolically illustrated as a simple cylindrical body with the resultant axial force of each FPS machine resulting in vibration forces causing vibration which is often considerable.
FIGS. 3 and 4 illustrate prior art beta FPS machines. FIG. 3 diagrammatically illustrates a single beta FPS machine 10 with an axis 12 of reciprocation. The phase of its composite, resultant vibration force can be illustrated by an arrow and/or + or − symbols for purposes, in some situations, of comparison to the phase of other FPS machines.
In the prior art it is known that a pair of two identical beta FPS machines can be positioned coaxially (coaxial axes of reciprocation) in an end to end relationship although they can have space between the ends. This prior art arrangement is illustrated in FIG. 4. There are two identical FPS machines 14 and 16 mounted coaxially along a common axis 18. The two machines 14 and 16 are physically oriented so they are in a mechanically opposed orientation, but they are operated thermodynamically in phase. Because they are mechanically opposed, the expansion spaces or alternatively the compression spaces of both are near (proximal) the center of this arrangement. The other space of each is at the opposite ends.
Because the FPS machines are mechanically opposed but operated thermodynamically in phase, the reciprocating masses of each machine move in the opposite direction from the corresponding masses of the other machine. Therefore, the resultant vibration forces of each machine are equal and opposite and cancel to eliminate or at least minimize net vibration. Of course multiple replications of this arrangement can be combined and also provide a balanced group.
There are several ways known in the prior art for controlling the relative thermodynamic phasing of two or more associated FPS machines. The relative phasing of their operation is controlled by their physical connections and structural characteristics. A simple example known in the prior art is that each FPS machines can be an engine connected to drive a linear alternator. Connecting such alternators together in the same polarity, forces the FPS engines to run in phase. Connecting such alternators together in opposite polarity forces the FPS engines to run in anti-phase. Therefore, for a group of 4 machines, all four linear alternators can be electrically connected together in parallel with two connected at the same polarity and the other two connected at a polarity opposite to the first two. Similarly, for a group of 6 machines, three may be connected in one polarity and three in the opposite polarity giving the result that the three FPS machines of each subgroup will run in phase with each other and in anti-phase to the other three FPS machines of the other subgroup. The same parallel connection for forcing phase relationships can be accomplished with linear motors driving FPS coolers. Other prior art means for forcing the two FPS machines to operate at a selected phase relationship include fluid couplings and thermodynamic cycle couplings. A connection from the inner end of one acceptor to the opposite engine's expansion space in an opposed pair forces the desired equal motions of displacers that uses the gas cycle as a forcing link. The gas from the acceptor of one engine must go to the other engine's expansion space. Two beta FPS machines can be forced to run in phase by connecting their expansion spaces together by a tube or passageway.
Consequently, the thermodynamic phase of operation of two or three beta FPS machines is not merely their manner of operating. It is the result of their structure and connection as known in the prior art. This is like a storage battery in the sense that the polarity of a storage battery, which determines the direction it pushes electrons through the external circuit, is not merely its manner of operation but rather is a characteristic of the machine that is a result of its structure, including its chemical structure. Because the structural characteristics of beta FPS machines that determine the relative thermodynamic phase of their operation is known in the prior art, it is not further described. The thermodynamic phase of each FPS machine may be viewed as and indicated by a polarity.
As described above and known in the prior art, an arrangement of two coaxially positioned beta FPS machines that are in a mechanically opposed orientation and operating in thermodynamically synchronous phase cancels vibration forces. However, if two beta FPS machines are not positioned coaxially, they either form a couple or they have a net translational vibration force. A couple is two parallel forces that are equal in magnitude but opposite in direction. A couple applies a torque to the entire composite mass of the machines which results in a vibrational torque.
The problem with FPS machines that are on non-coaxial axes of reciprocation is illustrated in FIGS. 15-18 for parallel axes of reciprocation. When the axes are neither parallel nor coaxial, the problem is made more complicated by the effect of the oblique resultants of the net vibrational forces and couples. In FIGS. 15-18 the E and C represent the expansion space end and the compression space end of the beta FPS machines and therefore represent the mechanical orientation of the machines. Referring to FIG. 15, if the axes of two parallel FPS machines are in a mechanically opposed orientation, and are operated in thermodynamically opposite reciprocation, their reciprocating masses move in mechanical synchronism and therefore they have a net vibrational translation force. Referring to FIG. 16, if the axes of two parallel FPS machines are in a mechanically opposed orientation, and are operated in thermodynamically synchronous reciprocation, their reciprocating masses move in mechanically opposed directions and they have a net vibrational couple and therefore a net vibrational torque. Referring to FIG. 17, if the axes of two parallel FPS machines are in a mechanically co-directional orientation, and are operated in thermodynamically opposite reciprocation, their reciprocating masses move in mechanically opposed directions and they have a net vibrational couple and therefore a net vibrational torque. Referring to FIG. 18, if the axes of two parallel FPS machines are in a mechanically co-directional orientation, and are operated in thermodynamically synchronous reciprocation, their reciprocating masses move in mechanical synchronism and therefore they have a net vibrational translation force.
The main purpose of the invention is to position and orient each beta, free piston Stirling machine of a group of beta, free piston Stirling machines in arrangements other than end to end coaxially and still cancel all the vibration forces and vibration torques that result from the acceleration and deceleration of their internal reciprocating masses. In other words, the sum of all acceleration forces (F=ma) from all reciprocating components and the sum of all couples (torque) both sum to zero. The arrangements that embody the invention provide groups of beta FPS machines that have a different aspect ratio than the long thin arrangement that characterizes the end to end coaxial arrangement while still canceling all force and torque vibrations. Different aspect ratios are preferred for different applications or implementations of FPS machines. For some applications or implementations of multiple FPS machines, it is desirable to have the machines in a long thin arrangement. For those applications, the prior art arrangement for canceling vibration forces is preferred. However, for some applications it is desirable to have an arrangement in which the FPS machines are more nearly or completely side by side so that the arrangement is more compact and not long and thin.
Another advantage of the present invention is that, unlike the end to end coaxial arrangements of the prior art, arrangements that embody the invention also allow the hot ends and/or the cold ends of such machines to be placed in nearby adjacent or laterally spaced positions. For example, the ends that accept heat can be conveniently located near the source of heat and/or the heat rejecting ends can be located near a heat sink. An example of this location of the respective ends is true for the examples of FIGS. 17 and 18, although they are not balanced because they do not embody the invention.
Yet another advantage of the present invention arises because the inventors believe that in the future, for some applications, multiple smaller beta FPS machines in a group will be a preferable implementation than a single or a few larger machines. Smaller machines are much less expensive to construct. Therefore, in some cases, economies of scale and mass production are likely to give a lower cost final product when comprised of multiple smaller machines.